The invention presented herein relates to digital communications systems of the type having a commutating N:1 multiplexer and a commutating 1:N demultiplexer which are operatively connected wherein N sources of digital signals are multiplexed and demultiplexed, each of the N sources providing signal bits at the rate of T bits/second with only one type of logic signals presented when no information is transmitted. It, in particular, relates to the circuitry for providing automatic frame synchronization and detection of frame synchronization in such systems.
Frame synchronization required in digital communication systems of the type related to the present invention has generally been attained by the use of special digital framing codes which are inserted at the multiplexer end of the system and are detected at the multiplexer end of the system for attaining frame synchronization. Such an approach to the frame synchronization problem requires the use of complex electronic circuitry at both the multiplexer and demultiplexer.